In recent years, the need for a cleaner atmosphere has become apparent. Manufacturers of vehicles powered by internal combustion engines, in the furtherance of this societal need, have undertaken a major effort to reduce the levels of undesirable emissions present in vehicle exhaust.
Oxides of nitrogen NOx, one of the many components in internal combustion engine emissions, has received much attention in emission control efforts. A common method for reducing NOx is recirculation of engine exhaust gas back into the air inlet of the engine to be combined with the incoming air charge, a process often called charge dilution or exhaust gas recirculation. By feeding the engine a combination of fresh inlet air and exhaust gas, the heat absorbing capacity of the air charge is increased and the overall oxygen content of the air charge is decreased. Increasing the heat absorbing capacity holds down engine combustion temperature, thereby inhibiting NOx formation. Decreasing oxygen content also decreases NOx formation by reducing the availability of one of its constituent elements.
In general, the more exhaust gas combined with the inlet air charge, the lower the production of NOx. However, beyond some point, the recirculation of exhaust gas increases the engine's emission of other pollutants in an unacceptable manner. Controlling exhaust gas recirculation EGR to achieve acceptable levels of all emission components while maintaining acceptable vehicle driveability is an increasingly difficult problem as emission constraints tighten and as vehicle operators expect more driveable vehicles.
Methods for controlling EGR according to predetermined engine operating conditions are in the prior art. Systems are available that admit an amount of exhaust gas into the air inlet according to such parameters as engine speed, engine temperature or engine intake manifold absolute pressure. Such systems sense these engine parameters and open an EGR valve, admitting exhaust gas into the inlet air charge in a quantity related to the sensed parameters. Such systems operate under the assumption that the relationship between these engine parameters will remain constant over the operational life of the system.
These systems achieve some success in NOx reduction, but their performance decays as the discussed relationships change. Over the life of an engine, system voltages can drift, affecting the integrity of sensed engine parameters. Additionally, engine parameters and their relationship to each other can change, and sensor accuracy may deteriorate. As traditional EGR systems are based on predetermined relationships between these sensed parameters, their performance is necessarily limited by the integrity of the sensors, the constancy of the supply voltages and the constancy of the relationships.